1. Field of the Invention
The present invention relates to a method of allocating thrusts in air and water craft (hereinafter referred to as craft) such as a ship and an airship by calculating manipulated variables to be given to a plurality of thrust generators equipped in the craft to allow the craft to generate a desired thrust, and a thrust allocating device for use in the method.
2. Description of the Related Art
Commonly, air and water craft such as a ship including a car ferry and a submarine, and an airship, is equipped with a plurality of thrust generators (thrusters). In order for the craft to run or be held at a fixed point, it is necessary to drive the plurality of thrust generators. In this case, there are several combinations of thrusts generated by the thrust generators to generate one thrust in the entire craft. In order to minimize fuel consumption in the thrust generators, allocation of the generated thrusts to the thrust generators becomes important. To meet such requirement, various methods of allocating the thrusts generated by the thrust generators have been proposed for the purpose of generating a targeted thrust in the entire craft.
One such method is disclosed in Japanese Laid-Open Patent Application Publication No. Hei. 1-311997. This thrust allocating method is to calculate thrusts generated by the thrust generators that satisfy force balance and moment balance (restricting condition) between the thrusts generated by the thrust generators and the thrust in the entire craft and minimize a value of a predetermined performance index associated with the thrusts generated by the thrust generators.
Japanese Laid-Open Patent Application No. Hei. 2001-219899 discloses a thrust allocating method in which, in craft equipped with turnable thrust generators, thrusts generated by the thrust generators that meet limitation of rated thrusts and operational angle ranges (turning angle ranges) of the thrust generators and minimize energy consumption are calculated. In this thrust allocating method, the thrusts generated by thrust generators that satisfy force balance and moment balance and minimize a value of a performance index associated with the thrusts generated by the thrust generators are calculated, and when the calculated thrusts are larger than the rated thrusts, a parameter corresponding to the associated thrust generator is changed and the thrust is re-calculated, or when there is a thrust generator whose azimuth is within a prohibition angle range, the azimuth is set outside the prohibition angle range.
Japanese Laid-Open Patent Application Publication No. Hei. 9-2393 discloses a thrust allocating method in which, in craft equipped with turnable thrust generators, thrusts generated by the thrust generators are calculated so as to minimize turning angles of the thrust generators. This thrust allocating method is to calculate the thrusts generated by the thrust generators that minimize the turning angles of the thrust generators by minimizing a value of a performance index associated with the turning angles of the thrust generators while keeping force balance and moment balance.
However, in the above-mentioned conventional thrust allocating method, the force balance and moment balance, i.e., the restricting condition, is not determined based on non-linear characteristics or dynamic characteristics of the thrust generators. Commonly, the thrust generators exhibit non-linear characteristics. One example is that, when the thrust generator is a propeller, the relationship between a rotational speed of the propeller and the corresponding generated thrust is non-linear. When the propeller is driven at a rotational speed (manipulated variable) proportional to the thrust derived from calculation in order to generate such thrust, there causes a large error between a targeted thrust and an actually generated thrust. For this reason, the conventional thrust allocating method is incapable of controlling the thrusts generated by the thrust generators with high precision.
In addition, the thrust generator has some dynamic characteristics such as delay. This would occur in the thrust generator provided with a rudder which responds at a relatively slow speed. In such a thrust generator, when the rudder is driven so that that the rudder angle is changed to generate the thrust derived from the calculation, it takes time to change the rudder to a desired rudder angle. Also, in this case, the conventional thrust allocating method is incapable of controlling the thrust generated by the thrust generator with high precision.
The above-mentioned performance index commonly involves weighting factors by which a state variable representing a state of the thrust generator and/or a manipulated variable of the thrust generator is to be multiplied. In the event of a failure in any of the thrust generators, it is necessary to set the thrust generator with the failure not to operate by, for example, changing the weighting factor associated with the thrust generator, and allocate thrusts to the remaining thrust generators. By doing so, a targeted thrust in the entire craft is correctly gained. However, in the conventional thrust allocating method, the weighting factor cannot be changed on line, i.e., in real time, and therefore when the failure occurs in the thrust generator, the thrusts generated by the remaining thrust generators cannot be correctly gained. Consequently, thrust allocation is substantially impossible.